Syrups making use of methyl methacrylate as a principal component of their starting monomers have been used for molding applications such as plates and boards reinforced with glass fibers, kitchen counter tables, sink cabinets and so on. They have very good properties in weathering resistance, including resistance to yellowing. Under severe use conditions such as immersion in boiling water for a prolonged period of time while being filled with such a filler as glass fibers, siliceous sand, alumina or the like, the resultant moldings were however accompanied by such drawbacks that they underwent whitening, delustering, etc. The present inventors have succeeded in leapingly improving the water resistance under severe conditions by mixing a syrup containing functional groups having reactivity with glycidyl groups and another syrup containing glycidyl groups in a stirrer-equipped tank, and using a catalyst such as benzoyl peroxide, azobisisobutyronitrile, t-butyl peroxypivalate, t-butyl peroxy-2-ethylhexanoate to cause the syrups to undergo radical polymerization while at the same time, allowing an addition reaction to proceed between the glycidyl groups and the functional groups having reactivity with the glycidyl groups.